Once you or they move away, they will easily forget about you and act like you don't exist. I have moved several times to different cities and even though we kinda keep in touch it really does feel like I am starting over with building a new social circle.

But even with long time close friends as soon as life gets busy they will stop putting in the effort to talk to you. There never is going to be a perfect time to stay in touch with someone because that is not how life works. Sometimes, you have to make time. You cannot take these connections for granted. When you get on your deathbed, you won't be thinking about all those hours you put in at work. You will be thinking about the relationships you had or could have had.